The present invention relates to centrifugal pumps of the pitot type and, more in particular, to an improved pitot pickup for such pumps that is particularly resistant to particle erosion.
Centrifugal pumps of the pitot type are well known. In general, these pumps include a drive that drives a rotor in rotation within a casing. A pitot pickup in a chamber of the rotor is stationary relative to the rotor and intercepts fluid within the chamber and draws that fluid from the chamber. The exiting fluid has a head larger than its inlet fluid head because of energy imparted to it by the rotor. Typically, fluid enters the rotor chamber through an annulus surrounding a pitot tube mounting tube and through a plurality of radial passages in a rotor chamber cover to exit near the outer radial limit of the chamber. The pitot inlet in the chamber may be comparatively close to the outer radial limits of the chamber, or comparatively close to the axis of rotation of the rotor, or both, depending on the application.
Centrifugal pumps of the pitot type find an application in petroleum recovery. Petroleum wells quite often employ downhole machinery to pump fluid produced by the well and fluid introduced into the well from the surface. These two fluids as pumped collectively are known as a power production fluid. The introduced fluid may be used to power downhole machinery. Typically, the downhole machinery is either a centrifugal or reciprocal engine. Power fluid for a downhole engine should be comparatively free of abrasives so that the engine is not damaged. For example, solid abrasives can erode journals and journal bearings of centrifugal machinery, in time erode blades of centrifugal machinery, and destroy seals in both types of machinery. Clearly, it is undesirable to subject downhole machinery to such a ruinous environment. Accordingly, attempts have been made to remove abrasives from the power fluid.
The problem of abrasive removal from power fluid is a continuous one. The power fluid is typically oil recovered from the well and may be in an open system where power fluid is continuously generated from well fluid. Solid contaminants abound in well fluid. Even in closed systems where power fluid circulates in a closed loop, makeup fluid for the circuit brings into it abrasive solids, and inevitable contamination with abrasive solids occurs from other sources, such as contaminated lines and leakage.
The well fluid may be multiple phase, having gas, oil and water content. Power fluid is usually single phase, either oil or water.
Centrifugal pitot pumps, usually in conjunction with other separating equipment, have been used to remove solids from power fluid streams and to separate well fluid into its phases. One technique for solid removal uses the fact that the solids are denser than fluid and employs radial exit ports in the rotor through which the solid contaminants exit the rotor chamber. Agitation vanes may be used to prevent buildup and blockage of these radial ports. When used to separate production fluid into its phases, pickups at different radii for each phase draw such phases from the rotor. These pickups may be pitot tubes.
The separation of solids from a production fluid stream through nozzles in the rotor of a pitot separator has disadvantages. When the casing outside the rotor is at atmospheric pressure, the fluid and solids entering the casing lose all their dynamic and pressure head. The fluid and solids in the typical case cannot be discharged into a collection vessel at the well. This discharge must, instead, be pumped away to some central collection area. The pump must typically raise the head of the solid and liquid discharge to several hundred pounds per square inch, say, 300 or 400 p.s.i. Sometimes it is inconvenient to provide a pump for the discharge at the well. The head lost through the nozzles could have been used to introduce the solid and liquid discharge into a flow line.
One system for separation of solids from production fluid employs a settling tank and a pitot separator. The settling tank effects phase separation by time and gravity. The pitot separator takes power fluid, say oil, from the separator and removes more solids from the stream. The pitot separator discharges separated solids and this fluid carrier back into the separator. The separator operates at flow line pressure so the discharge from the pitot separator does not suffer the large head loss that the alternative method entailed. However, the casing pressure of the pitot separator was necessarily high, and mechanical seal problems resulted.
The pitot in the path of the rotating fluid within the chamber is subjected to the erosive effect of solids carried by the fluid. The erosive effect is directly proportional to the cube of the relative velocity between the fluid and the pitot. Particularly at comparatively large radii of the chamber, the erosive effect of the solids can be startling.
Known pitot pumps includes those described in the following U.S. Pat. Nos. 3,384,024; 3,776,658; 3,795,459; 3,817,659; 3,838,939; 3,926,534; 3,960,319; 3,977,810; and 3,994,618. The pitot pickups of these patents are generally characterized by a head that extends forward toward oncoming fluid from a body. The entrance to the pitot pickup is in the head, and the material of the head surrounding the entrance may be sharp. The body below the head typically has a sharp leading edge. The surface of the head behind the entrance where cut by radial planes from the axis of the rotor is streamlined.